1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for manufacturing the same. In particular, the present invention relates to a liquid crystal display device having liquid crystal molecules axially symmetrically aligned within a liquid crystal region partitioned by a polymer wall.
2. Description of the Related Art
Conventionally, a TN (twisted nematic) type or a STN (super-twisted nematic) type using a nematic liquid crystal is used as a display device employing an electro-optic effect. A technique of widening a viewing angle of such a liquid crystal display device has been developed.
As one suggested technique of widening a viewing angle of the TN-type liquid crystal display device, Japanese Laid-Open Publication Nos. 6-301015 and 7-120728 disclose a so-called ASM (Axially Symmetrically aligned Microcell) mode, in which a liquid crystal display device has liquid crystal molecules axially symmetrically aligned within a liquid crystal region partitioned by polymer walls. The liquid crystal region substantially surrounded by the polymer wall is typically formed for each picture element. In a liquid crystal display device of this mode, the liquid crystal molecules are axially symmetrically aligned, so that there are a few changes in contrast regardless of the direction in which a viewer sees a screen of the liquid crystal display device. Thus, it has a wider viewing angle.
The ASM-mode LCD disclosed by the aforementioned publications is produced by a polymerization induced phase separation of a mixture containing a polymerizable material and a liquid crystal material.
A method for manufacturing the conventional ASM-mode liquid crystal display device will be described with reference to FIGS. 10A to 10I. First, a substrate is prepared by forming a color filter and an electrode on one surface of a glass substrate 908 (FIG. 10A). Note that for simplicity the electrode and the color filter formed on the glass substrate 908 are not shown. A method for forming the color filter will be described later.
Next, on the surface of the glass substrate 908 where the electrode and the color filter are formed, polymer walls 917 for aligning liquid crystal molecules in axial symmetry is formed in a lattice pattern, for example (FIG. 10B). After a photosensitive resin material is spin-coated, it is exposed through a photomask having a predetermined pattern and developed. As a result, the polymer walls in a lattice pattern are formed. The photosensitive resin material may be either a negative-type or a positive-type. A step of forming an additional resist film is added, but it can be formed by using a resin material with no photosensitivity.
On a top portion of each of the resultant polymer walls 917, a pillar-like protrusion 920 is separately formed by patterning (FIG. 10C). Like the polymer walls 917, the pillar-like protrusions 920 are also formed by exposing and developing a photosensitive resin material.
The surface of the glass substrate 908 having the polymer walls 917 and the pillar-like protrusions 920 is coated with a vertical alignment agent 921 such as polyimide (FIG. 10D). Separately, a glass counter substrate 902 with an electrode formed thereon is also coated with a vertical alignment agent 921 (FIGS. 10E and 10F).
A liquid crystal cell is formed by attaching the two resultant substrates to each other so that the surfaces having the electrodes face each other (FIG. 10G). A gap between two substrates (i.e., a cell gap (a thickness of a liquid crystal layer)) is defined as a sum of the polymer wall 917 and the pillar-like protrusion 920.
A liquid crystal material is injected into a gap of the resultant liquid crystal cell by a vacuum injection method, for example (FIG. 10H). In the end, by applying a voltage between the opposing electrodes, for example, the crystal molecules within the liquid crystal region 916 are axially symmetrically aligned (FIG. 10I). The liquid crystal molecules within the liquid crystal region 916 partitioned by the polymer walls 917 are symmetrically aligned with respect to an axis 918 (perpendicular to both substrates) represented by a broken-line in FIG. 10I.
FIG. 11 is a cross-sectional view illustrating a structure of a conventional color filter. On a glass substrate 508, a black matrix (BM) 510 for blocking light from being transmitted through gaps between colored portions, and colored resin layers 512 of red, green, and blue (R, G, B) corresponding to respective picture element are formed. An over-coat (OC) layer 514 of acrylic resin, epoxy resin, or the like, with a thickness of about 0.5-2.0 .mu.m is formed over the BM 510 and the colored resin layer 512 in order to improve smoothness, for example. In addition, an indium tin oxide (ITO) layer 516, which is a transparent signal electrode, is formed on the OC layer 514. The BM is generally composed of metal chrome having a thickness of about 100-150 nm. Resin materials colored with dyes or pigments are used for the colored resin layer 512, and a thickness of this layer is generally about 1-3 .mu.m.
As a method for forming a color filter, a photosensitive colored resin layer formed on a substrate is patterned by using a photolithography method. For example, a red (R), green (G), and blue (B) color filter can be formed when photosensitive resin layers of respective colors are formed, exposed, and developed (three times in total) by employing the photosensitive resin materials of the respective colors. In order to form a photosensitive colored resin layer, a substrate may be spin-coated with a liquid photosensitive colored resin material (which is diluted with a solvent). Alternatively, a photosensitive colored resin material may be transferred onto the substrate in the form of a dry film. By manufacturing the aforementioned liquid crystal display device of the ASM mode by using the color filter, a color liquid crystal display device having a wide viewing angle characteristic can be obtained.
However, the inventors of the present invention have found the following problems in the conventional ASM-mode liquid crystal display device and the method for manufacturing the same.
In the conventional ASM-mode liquid crystal display device, while it is possible to obtain a wide viewing angle characteristic, the brightness of the display device is reduced because the polymer walls reduce the light transmission. Moreover, the liquid crystal molecules present on the polymer walls in the conventional liquid crystal display device cannot contribute to display of images, so that the transmittance of the liquid crystal display device is reduced. In addition, the axial symmetry alignment of the liquid crystal molecules in the vicinity of the polymer walls is disturbed, thereby causing flickers in images (e.g., light leakage) in a black display.
In addition, when the above-described ASM-mode liquid crystal display device and the method for manufacturing the same are applied to a plasma-addressed liquid crystal display, the following problems arise. In a plasma-addressed liquid crystal display device, a plasma cell portion and a liquid crystal cell portion, which form a switching portion, have different heat history during the manufacturing steps (typically, 500.degree. C. for the plasma cell, and 200.degree. C. for the liquid crystal cell). Accordingly, difference in dimension resulting from heat contraction between both cells also varies. Thus, it is difficult to exactly align plasma electrodes with ITO electrodes. In view of this, a structure which eliminates the need of such a difficult alignment process has been employed (also known as an alignment-free structure). When the plasma-addressed liquid crystal display device and the ASM-mode are combined with the alignment-free structure, polymer walls (conventionally made from a black material) which are often formed within the aperture of each pixel in order that liquid crystal molecules are aligned in axial symmetry. As a result, the aperture ratio is reduced in comparison to the conventional TN-type ASM-mode liquid crystal display device. This may further reduce the brightness of the display.
Furthermore, when black photosensitive resin is applied over the entire surface of a substrate for forming the polymer walls, alignment marks for aligning become difficult to see. As a result, it becomes impossible to achieve accurate alignment during the patterning process, which makes it impossible to form the polymer walls in desired positions. In order to solve this problem, the black photosensitive resin covering the alignment marks may be wiped off, although it makes the manufacturing process more complicated. As an alternative solution, it is possible to apply the black photosensitive resin so as not to cover the alignment marks. For example, the black photosensitive resin maybe selectively applied onto a substrate by employing a roll-coater. However, in comparison to the spin-coating method for applying the resin over the entire surface of the substrate, the method employing the roll-coater is inferior in production of a uniformly thick layer. As described above, it was difficult to form the polymer walls in the desired positions without making the manufacturing process more complicated or degrading the accuracy of the process.
Moreover, in the method for manufacturing the conventional ASM-mode liquid crystal display device, the polymer walls formed on the substrate may interfere with injection of the liquid crystal material in the gap of the liquid crystal cell. As a result, a time required to inject the liquid crystal material becomes longer, thereby decreasing throughput.